An example from the #StarTrek world is how TNG has been remastered from film but DS9 and VOY haven't, which is why ironically the older series looks better on your modern screen.
From what I gather, all three were shot on film, but the newer ones were edited at TV quality and had lots of digital effects that would also have to be remade.
Babylon 5 was remastered recently and they had the same issue as DS9 and VOY, but they went ahead and did the editing and I think the effects again. I would welcome a fact check on this.